High-intensity discharge (HID) lamp is featured by intense luminescence, long longevity, small size, and excellent illuminant efficiency. Thus, the High-intensity discharge lamps have been widely employed in outdoor situations or indoor situations, or used as the illuminating device for automobiles.
Generally, the high-intensity discharge lamp is mounted in a lamp seat that is durable under a high voltage of 5000V. Moreover, the high-intensity discharge lamp must be operated in cooperation with an electronic ballast. Referring to FIG. 1, which shows a circuit block diagram of a conventional electronic ballast. As shown in FIG. 1, the conventional electronic ballast 9 is used to excite the high-intensity discharge lamp Lp when the HID lamp is operating in the transient ignition stage, and provide a steady current for the high-intensity discharge lamp Lp when the high-intensity discharge lamp Lp is operating in the stable stage. The electronic ballast 9 includes a power circuit 90 and an ignition circuit 91. The power circuit 90 includes an AC/DC converter 900, a DC/DC converter 901, and an inverter 902. The AC/DC converter 900 is used to receive an AC voltage Vac and convert the AC voltage Vac into a first DC voltage V1′. The DC/DC converter 901 is used to convert the first DC voltage V1′ into a second DC voltage V2′. The inverter 902 is used to convert the second DC voltage V2′ into an operating AC voltage Vw′ for powering the high-intensity discharge lamp Lp when the high-intensity discharge lamp Lp is operating in the stable stage.
Referring to FIGS. 2 and 1, in which FIG. 2 is a circuit diagram showing the circuit structure of the ignition circuit of FIG. 1. The ignition circuit 91 is used to receive the power provided by the power circuit 90 and convert the power provided by the power circuit 90 into a high-level excitation voltage Vs′. The power provided by the power circuit 90 may be the first DC voltage V1′ outputted from the AC/DC converter 900 or the second DC voltage V2′ outputted from the DC/DC converter 901. When the high-intensity discharge lamp Lp is operating in the transient state, the excitation voltage Vs′ excites the high-intensity discharge lamp Lp. The ignition circuit 91 includes a switch element M and a transformer T′. The switch element M is connected in series with the primary winding Nf′ of the transformer T′, and the control terminal of the switch element M is used to receive a pulse signal (not shown). The secondary winding Ns′ of the transformer T′ is connected to the high-intensity discharge lamp Lp. When the pulse signal is in the enabling state and the switch element M is driven to turn on accordingly, the transformer T′ converts the power received by the primary winding Nf′ from the power circuit 90 and generates a high-level excitation voltage Vs′ across the secondary winding Ns′ to excite the high-intensity discharge lamp Lp. After the high-intensity discharge lamp Lp is excited, the pulse signal is transitioned to be in the disabling state or the pulse signal is stopped from being outputted to the control terminal of the switch element, thereby turning off the switch element M.
The ignition circuit 91 of the conventional electronic ballast 9 is able to excite the high-intensity discharge lamp Lp by the excitation voltage Vs′. Moreover, the pulse signal received by the switch element M of the ignition circuit 91 is a square wave and the time period for transitioning the pulse signal from the disabling state to the enabling state is very short. Therefore, the duration of the time period for transitioning the pulse signal d depends on the performance of the switch element M. Generally, the time period for transitioning the pulse signal from the disabling state to the enabling state is about tens of nanoseconds. However, the on-state time of the switch element M in the enabling state is tens of microseconds or longer. Hence, the transition of the switch element from the OFF state to the ON state will be considered instantaneous. In this way, the excitation voltage Vs′ indicated by the curve S2 of FIG. 3 will have a considerable voltage jitter A2′ as the switch element M is instantaneously transitioning from the OFF state to the ON state. Moreover, the peak voltage value A1′ of the excitation voltage is about 6 KV, which exceeds the default safe voltage value. For example, the default safe voltage value, i.e. the voltage durability of lamp seat, is 5 KV. Thus, the longevity of the high-intensity discharge lamp Lp is shortened, and the lamp seat used for housing the high-intensity discharge lamp Lp may be burned out. Also, the voltage jitter A2′ may not be able supply enough excitation energy to smoothly ignite the high-intensity discharge lamp Lp. In practical applications, the length of the output line connecting the electronic ballast 9 and the lamp seat may vary from case to case, and the parasite capacitance of the output line will affect the peak voltage value A1′ and the voltage jitter A2′ of the excitation voltage Vs′, thereby incurring safety problems or deteriorating the ignition effect.
Although other types of the ignition circuit, such as the ignition circuit 8 shown in FIG. 4 which additionally places a capacitor C′ connected in parallel with the discharge lamp Lp, or the ignition circuit 7 shown in FIG. 5 which additionally places an inductor L′ connected in series with the primary winding Nf′ of the transformer T′, are used to reduce the peak voltage value and voltage jitter of the excitation voltage Vs′ by the extrinsic capacitor C′ or the extrinsic inductor L′, the addition of the extrinsic element causes the dimensional enlargement of the electronic ballast or the ignition circuit and the increment of the manufacturing cost.
Hence, the inventors are mandatory to develop a method of controlling an ignition circuit and an electronic ballast applying such method to control the ignition circuit thereof, for the sake of resolving the aforementioned drawbacks and problems.